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Media Access

•  Motivation •  SDMA, FDMA, TDMA •  Aloha, reservation schemes •  Collision avoidance, MACA •  Polling •  CDMA, SAMA •  Comparison

Motivation   Can we apply media access methods from fixed

networks?   Example CSMA/CD

  Carrier Sense Multiple Access with Collision Detection   send as soon as the medium is free, listen into the medium if a

collision occurs (legacy method in IEEE 802.3)

  Problems in wireless networks   signal strength decreases proportional to the square of the

distance   the sender would apply CS and CD, but the collisions happen

at the receiver   it might be the case that a sender cannot “hear” the collision,

i.e., CD does not work   furthermore, CS might not work if, e.g., a terminal is “hidden”

Motivation - hidden and exposed terminals   Hidden terminals

  A sends to B, C cannot receive A   C wants to send to B, C senses a “free” medium (CS fails)   collision at B, A cannot receive the collision (CD fails)   A is “hidden” for C

  Exposed terminals   B sends to A, C wants to send to another terminal (not A or B)   C has to wait, CS signals a medium in use   but A is outside the radio range of C, therefore waiting is not

necessary   C is “exposed” to B

B A C

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Motivation - near and far terminals   Terminals A and B send, C receives

  signal strength decreases proportional to the square of the distance   the signal of terminal B therefore drowns out A’s signal   C cannot receive A

  If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer

  Also severe problem for CDMA-networks - precise power control needed!

A B C

Access methods SDMA/FDMA/TDMA   SDMA (Space Division Multiple Access)

  segment space into sectors, use directed antennas   cell structure

  FDMA (Frequency Division Multiple Access)   assign a certain frequency to a transmission channel between

a sender and a receiver   permanent (e.g., radio broadcast), slow hopping (e.g., GSM),

fast hopping (FHSS, Frequency Hopping Spread Spectrum)   TDMA (Time Division Multiple Access)

  assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

  The multiplexing schemes presented previously are now used to control medium access!

FDD/FDMA - general scheme Example GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

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TDD/TDMA - general scheme Example DECT

1 2 3 11 12 1 2 3 11 12

t downlink uplink

417 µs

Aloha/slotted aloha   Mechanism

  random, distributed (no central arbiter), time-multiplex   Slotted Aloha additionally uses time-slots, sending must always

start at slot boundaries

  Aloha

  Slotted Aloha

sender A

sender B

sender C

collision

sender A

sender B

sender C

collision t

t

DAMA - Demand Assigned Multiple Access   Reservation can increase efficiency to 80%

  a sender reserves a future time-slot   sending within this reserved time-slot is possible without

collision   reservation also causes higher delays   typical scheme for satellite links

  Examples for reservation algorithms:   Explicit Reservation according to Roberts (Reservation-

ALOHA)   Implicit Reservation (PRMA)   Reservation-TDMA

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Access method DAMA: Explicit Reservation   Explicit Reservation (Reservation Aloha):

  two modes:   ALOHA mode for reservation:

competition for small reservation slots, collisions possible   reserved mode for data transmission within successful reserved

slots (no collisions possible)   it is important for all stations to keep the reservation list

consistent at any point in time and, therefore, all stations have to synchronize from time to time

Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t

Access method DAMA: PRMA   Implicit reservation (PRMA - Packet Reservation MA):

  a certain number of slots form a frame, frames are repeated   stations compete for empty slots according to the slotted aloha

principle   once a station reserves a slot successfully, this slot is automatically

assigned to this station in all following frames as long as the station has data to send

  competition for this slots starts again as soon as the slot was empty in the last frame

frame1

frame2

frame3

frame4

frame5

1 2 3 4 5 6 7 8 time-slot

collision at reservation attempts

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F D t

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

Access method DAMA: Reservation-TDMA   Reservation Time Division Multiple Access

  every frame consists of N mini-slots and x data-slots   every station has its own mini-slot and can reserve up to k

data-slots using this mini-slot (i.e. x = N * k).   other stations can send data in unused data-slots according to

a round-robin sending scheme (best-effort traffic)

N mini-slots N * k data-slots

reservations for data-slots

other stations can use free data-slots based on a round-robin scheme

e.g. N=6, k=2

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MACA - collision avoidance   MACA (Multiple Access with Collision Avoidance) uses

short signaling packets for collision avoidance   RTS (request to send): a sender request the right to send from

a receiver with a short RTS packet before it sends a data packet

  CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

  Signaling packets contain   sender address   receiver address   packet size

  Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

MACA examples   MACA avoids the problem of hidden terminals

  A and C want to send to B

  A sends RTS first   C waits after receiving

CTS from B

  MACA avoids the problem of exposed terminals   B wants to send to A, C

to another terminal   now C does not have

to wait for it cannot receive CTS from A

A B C

RTS

CTS CTS

A B C

RTS

CTS

RTS

MACA variant: DFWMAC in IEEE802.11

idle

wait for the right to send

wait for ACK

sender receiver

packet ready to send; RTS

time-out; RTS

CTS; data

ACK

RxBusy

idle

wait for data

RTS; RxBusy

RTS; CTS

data; ACK

time-out ∨ data; NAK

ACK: positive acknowledgement NAK: negative acknowledgement

RxBusy: receiver busy

time-out ∨ NAK; RTS

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MACA: In practice not used…   Most current applications use base stations (not ad-hoc

networks)   What does usual traffic look like and what’s the cost of

RTS/CTS?   802.11 end-user documentation recommends disabling

RTS/CTS “unless you are experiencing unusually poor performance”

  In 802.11, interference range often more than double communication range

  Studies show RTS/CTS does not improve throughput in 802.11 multi-hop networks

  Drivers leave it off by default

Polling mechanisms   If one terminal can be heard by all others, this “central”

terminal (a.k.a. base station) can poll all other terminals according to a certain scheme   now all schemes known from fixed networks can be used

(typical mainframe - terminal scenario)   Example: Randomly Addressed Polling

  base station signals readiness to all mobile terminals   terminals ready to send can now transmit a random number

without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address)

  the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address)

  the base station acknowledges correct packets and continues polling the next terminal

  this cycle starts again after polling all terminals of the list

ISMA (Inhibit Sense Multiple Access)   Current state of the medium is signaled via a “busy

tone”   the base station signals on the downlink (base station to

terminals) if the medium is free or not   terminals must not send if the medium is busy   terminals can access the medium as soon as the busy tone

stops   the base station signals collisions and successful transmissions

via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach)

  mechanism used, e.g., for CDPD (integrated into AMPS)

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Access method CDMA   CDMA (Code Division Multiple Access)

  all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel

  each sender has a unique random number, the sender XORs the signal with this random number

  the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function

  Disadvantages:   higher complexity of a receiver (receiver cannot just listen into the

medium and start receiving if there is a signal)   all signals should have the same strength at a receiver

  Advantages:   all terminals can use the same frequency, no planning needed   huge code space (e.g. 232) compared to frequency space   interferences (e.g. white noise) is not coded   forward error correction and encryption can be easily integrated

CDMA in theory   Sender A

  sends Ad = 1, key Ak = 010011 (assign: “0”= -1, “1”= +1)   sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

  Sender B   sends Bd = 0, key Bk = 110101 (assign: “0”= -1, “1”= +1)   sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

  Both signals superimpose in space   interference neglected (noise etc.)   As + Bs = (-2, 0, 0, -2, +2, 0)

  Receiver wants to receive signal from sender A   apply key Ak bitwise (inner product)

  Ae = (-2, 0, 0, -2, +2, 0) • Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6   result greater than 0, therefore, original bit was “1”

  receiving B   Be = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0”

CDMA on signal level I

data A

key A

signal A

data ⊕ key

key sequence A

Real systems use much longer keys resulting in a larger distance between single code words in code space.

1 0 1

1 0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As

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CDMA on signal level II

signal A

data B

key B key

sequence B

signal B

As + Bs

data ⊕ key

1 0 0

0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

CDMA on signal level III

Ak

(As + Bs) * Ak

integrator output

comparator output

As + Bs

data A

1 0 1

1 0 1 Ad

CDMA on signal level IV

integrator output

comparator output

Bk

(As + Bs) * Bk

As + Bs

data B

1 0 0

1 0 0 Bd

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comparator output

CDMA on signal level V

wrong key K

integrator output

(As + Bs) * K

As + Bs

(0) (0) ?

  Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time

  Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha

SAMA - Spread Aloha Multiple Access

1 sender A 0 sender B

0 1

t

narrow band

send for a shorter period with higher power

spread the signal e.g. using the chipping sequence 110101 (“CDMA without CD“)

Problem: find a chipping sequence with good characteristics

1 1

collision

Comparison SDMA/TDMA/FDMA/CDMA